Many transmembrane receptors such as G protein-coupled receptors (GPCRs), transporters, and ion channels exist in many interconvertible three-dimensional conformations depending on their activity or ligand-binding state. Agents that specifically bind to a transmembrane receptor in a conformationally-specific way can be used to induce a conformational change in the transmembrane receptor. Such agents have therapeutic applications and can be used in X-ray crystallography studies of the transmembrane receptor.